Message-ID: <530B449C.8030205@iogearbox.net>
Date: Mon, 24 Feb 2014 14:09:48 +0100
From: Daniel Borkmann <borkmann@iogearbox.net>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0
MIME-Version: 1.0
To: "H. Peter Anvin" <hpa@zytor.com>
CC: Jan Beulich <JBeulich@suse.com>, Daniel Borkmann <dborkman@redhat.com>,
        davem@davemloft.net, mingo@elte.hu, tglx@linutronix.de,
        ffusco@redhat.com, tgraf@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] x86/hash: swap parameters of crc32_u32()
References: <53073986020000780011E2E1@nat28.tlf.novell.com> <5308938A.8070506@redhat.com> <530B0AF2020000780011E97B@nat28.tlf.novell.com> <530B1D60.8010602@redhat.com> <530B32A4020000780011EACB@nat28.tlf.novell.com> <530B3102.4050102@redhat.com> <530B3BDB.3010908@zytor.com> <530B4C0C020000780011EC0D@nat28.tlf.novell.com> <530B4058.80705@zytor.com> <530B429C.2090507@zytor.com>
In-Reply-To: <530B429C.2090507@zytor.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org

On 02/24/2014 02:01 PM, H. Peter Anvin wrote:
> On 02/24/2014 04:51 AM, H. Peter Anvin wrote:
>> On 02/24/2014 04:41 AM, Jan Beulich wrote:
>>>>
>>>> So I'm guessing this hash is deliberately using the CRC32 instruction
>>>> "backwards", which would actually make sense: an actual CRC is actually
>>>> a pretty poor hash due to linearity.
>>
>> OK, it really is even more confusing than that.
>>
>> It does seem like the crc32 instruction really *is* commutative, which
>> isn't something I would personally have expected at all.
>>
>> Given that fact, I suspect the ordering in the DPDK is actually a bug,
>> and that we should correct the ordering (which I would do at the call
>> sites because it seems to make the code clearer) because it reduces the
>> size of the loop by two instructions.
>>
>> I guess I should find out how to file a bug report against DPDK too...
>
> Looking through the DPDK project git history, it seems that this was a bug introduced when changing from using inline assembly to using intrinsics:
>
>   static inline uint32_t
>   rte_hash_crc_4byte(uint32_t data, uint32_t init_val)
>   {
> -    asm volatile("crc32 %[data], %[init_val]"
> -                 : [init_val]"=r" (init_val)
> -                 : [data]"r" (data), "[init_val]" (init_val));
> -    return init_val;
> +    return _mm_crc32_u32(data, init_val);
>   }

Good point, I also just noticed that in the git blame.

> The operand order, of course, of the intrinsic being the opposite of AT&T-style assembly.
>
> I never expected that the CRC32 operation would be commutative.  Very fascinating.

Indeed.

>      -hpa
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/